In recent years, light-emitting-device type display devices (light emitting devices) including a display panel (pixel arrays) having current-driven light emitting elements arranged in a matrix manner are getting attention as next-generation display devices. Examples of such current-driven light emitting element are an organic electro-luminescence device (organic EL device), a non-organic electro-luminescence device (non-organic EL device), and a light emitting diode (LED).
In particular, light-emitting-device type display devices with an active-matrix driving scheme have a faster display response speed in comparison with conventionally well-known liquid crystal display devices, have little view angle dependency, and have a good display characteristic which enable accomplishment of high brightness, high contrast, and high definition of a display quality. The light-emitting-device type display devices need no backlight and light guiding plate unlike the liquid crystal display devices, and have a superior advantage that the light-emitting-device type display devices can be further thinned and light-weighted. Therefore, it is expected that such display devices are applied to various electronic devices in future.
For example, Unexamined Japanese Patent Application KOKAI Publication No. H08-330600 discloses an organic EL display device which is an active-matrix drive scheme display device that is subjected to a current drive by a voltage signal. In such an organic EL display device, a circuit (referred to as a “pixel driving circuit” for descriptive purpose) including a current driving thin-film transistor and a switching thin-film transistor is provided for each pixel. The current driving thin-film transistor allows a predetermined current to flow through an organic EL device that is a light emitting element as a voltage signal according to image data is applied to the gate of such a transistor. Moreover, the switching thin-film transistor performs a switching operation in order to supply the voltage signal according to image data to the gate of the current driving thin-film transistor.
According to such an organic EL display device that controls the brightness and gradation of the light emitting element based on a voltage signal, however, when a threshold voltage of the current driving thin-film transistor or the like changes with time, the current value of a current flowing through the organic EL device becomes varied.
Moreover, in the pixel driving circuits for respective plural pixels arranged in a matrix manner, even if respective threshold voltages of the current driving thin-film transistors remain same, varying of the gate insulation film, the channel length, and the mobility of the thin-film transistor affect the driving characteristic, which results in varying thereof.
It is known that varying in the mobility remarkably occurs especially in the case of a low-temperature polysilicon thin-film transistor. If an amorphous silicon thin-film transistor is used, the mobility can be uniform but a negative effect by such varying originating from a manufacturing process is inevitable.